Electrochemical measuring sensor and method for producing an electrochemical measuring sensor

ABSTRACT

An electrochemical measuring sensor having a solid electrolyte, a first electrode exposed to a gas to be measured and a second electrode exposed to a reference gas, with the electrodes preferably being arranged on opposite sides of the solid electrolyte. At least one of the electrodes (16, 20) is provided with a contouring (24) on its side (22) that is exposed to the gas to be measured or to the reference gas, with the contouring being a trench-shaped groove (26) embossed into the surface of the electrode such that the electrode is pressed into the adjacent solid electrolyte in the region of the groove. According to the method, the embossing is done while the electrode and the solid electrolyte are in the non-sintered state, and the sensor is subsequently sintered.

This application is a 371 of PCT/DE96/01753 filed Sep. 17, 1996.

The invention relates to an electrochemical measuring sensor having asolid electrolyte, a first electrode exposed to a gas to be measured anda second electrode exposed to a reference gas, with the electrodespreferably being arranged on opposite sides of the solid electrolyte,and to a method of producing an electrochemical measuring sensor havinga solid electrolyte, a first electrode exposed to a gas to be measuredand a second electrode exposed to a reference gas, wherein theelectrodes are applied essentially layer-shaped on the solid electrolyteand the measuring sensor is subsequently sintered.

STATE OF THE TECHNOLOGY

Electrochemical measuring sensors of the generic type are known. As arule, these have a layered structure wherein a solid electrolyte, whichsimultaneously acts as substrate, is provided with respectively oneelectrode on opposite sides. One of the electrodes is exposed to a gasto be measured and the other electrode to a reference gas, usually toatmospheric air. Corresponding to an oxygen content in the gas to bemeasured, a specific partial oxygen pressure appears at the electrodefacing the gas to be measured. This partial oxygen pressure is at aspecific ratio with respect to the partial oxygen pressure coming fromthe reference gas and appearing at the electrode facing the referencegas. On the basis of the resulting difference in oxygen concentration atthe electrodes, a specific detector voltage occurs between theelectrodes, which detector voltage can be evaluated by way of a suitableevaluation circuit and therewith supplies a signal corresponding to theoxygen concentration present at the electrode exposed to the gas to bemeasured. A chemical measuring sensor of this type is known, forexample, from DE-OS 29 28 496, corresponding to U.S. Pat. No. 4,294,679.Here, the electrode that is exposed to the reference gas is providedwith a cover. The side of the cover facing the electrode has trench-likepatterns which permit a feeding of the reference gas to the electrode.Thus, the chemical measuring sensor has a design comprised of relativelymany individual layers which are fixedly bonded to one another by meansof a generally known sintering process. The drawback of the known designof the electrochemical measuring sensor is that the effective electrodesurface, which is in direct contact with the reference gas, isrelatively small compared to the actual electrode surface.

SUMMARY AND ADVANTAGES OF THE INVENTION

The electrochemical measuring sensor according to the invention offersthe advantage that a relatively large effective electrode surface isavailable. Due to the fact that at least one of the electrodes has acontouring on its side exposed to the gas to be measured or to thereference gas, it is possible in a simple manner to make the electrodesurface larger while the outer size of the electrochemical measuringsensors remains unchanged. Because of the contouring, preferably formedby trench-shaped patterns, the electrode surface of the electrode can bemade larger so that a correspondingly higher electrode activity, forexample, a higher pumping output, of the electrode is available.

A preferred embodiment of the invention provides that the contouring isformed by trench-shaped patterns resulting in a network pattern, whichpatterns are used as reference gas channels. This accomplishes in a veryadvantageous manner that, due to the contouring of the electrode itselfthat is exposed to the reference gas, the arrangement of an additionallayer of the electrochemical measuring sensor, the layer being providedwith the reference air channels, is no longer necessary. Thus, thedesign of the electrochemical measuring sensor can be simplified.Furthermore, a miniaturization of the electrochemical measuring sensoris possible because an additional layer is eliminated.

Furthermore, the method according to the invention for producing anelectrochemical measuring sensor offers the advantage that, in a mannerthat is simple and suitable for mass production, electrochemicalmeasuring sensors can be produced which are characterized by a simpleand robust construction. Since at least one of the electrodes iscontoured before sintering on its side exposed to the gas to be measuredor the reference gas, it is advantageously possible, on the one hand, toutilize the contouring for an enlargement of the effective electrodesurface and, on the other hand, to accomplish a greater mechanicalstability of the electrode or of the measuring sensors provided with theelectrode due to the contouring, so that the handling of the measuringsensors during both the production process and the mounting into asensor element is improved.

It is particularly advantageous that the electrodes are embossed to formthe contouring. Applying the embossing on the electrode prior to thesintering of the electrochemical measuring sensor is possible in asimple manner by means of a corresponding embossing stamp at a moment atwhich the electrode or the measuring sensor has not yet been sintered,but at which these are present as so-called green films. Therewith,these have a good deformation ability so that highly precise contouringcan be accomplished by means of the embossing, which contouring remainsintact after the electrochemical measuring sensor has been sintered.

Further advantageous embodiments of the invention result from theremaining features cited in the dependent claims.

DRAWINGS

The invention is explained below in greater detail by way of embodimentswith reference to the associated drawings wherein:

FIG. 1 is a schematic sectional representation through anelectrochemical measuring sensor;

FIG. 2 is a perspective plan view onto an electrode exposed to thereference gas; and

FIG. 3 a plan view onto an electrode exposed to a gas to be measuredaccording to a further embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an electrochemical measuring sensor, generally identifiedby 10, which sensor can be used, for example, for determining the oxygencontent in gas mixtures, particularly in exhaust gases of internalcombustion engines. The measuring sensor 10 is comprised of a solidelectrolyte 12 on whose side 14, here shown at the top, a firstelectrode 16 is arranged. A second electrode 20 is arranged on the otherside 18 of the solid electrolyte 12. Here, the electrode 20 is embeddedin the solid electrolyte 12, so that an outer side 22 of the electrode20 is flush with the side 18 of the solid electrolyte 12 and overallresults in a planar surface.

Seen in cross section, the electrode 20 has a meander-shaped coursewhose design will be explained below in greater detail. The electrode 20has a contouring 24 which is formed by trench-shaped patterns 26 whichare open towards the outer side 22. As will be seen by way of FIG. 2,the trench-shaped patterns 26 form a network 28 in which trench-shapedpatterns 26 that extend longitudinally to the electrode 20 crosstrench-shaped patterns 26 that are arranged transversely to theelectrode 20.

The side 18 of the solid electrolyte 12 is provided with a cover plate30. The cover plate 30 closes the trench-shaped patterns 26 on the outerside 22 of the electrode 20, thus forming a branched channel system. Thetrench-shaped patterns 26 are thus delimited on three sides by theelectrode 20 and, on their fourth side, by the cover plate 30.Optionally, a heating device 32 can be arranged in the cover plate 30,in which heating device heating conductors 36 are arranged in a layer34.

The trench-shaped patterns 26 are open on one side of the measuringsensor 10, in particular on an end face of the measuring sensor 10, sothat a reference gas can get to the electrode 20 through thenetwork-type channel system formed by the trench-shaped patterns 26.Since the trench-shaped patterns 26 are surrounded by the electrode 20on three sides, an effective surface of the electrode 20, which comesinto direct contact with the reference gas, is relatively large. In theexample that is shown, this relative surface electrode 20 is three timeslarger than a conventional electrode that is applied on the solidelectrolyte 12 in a planar manner.

The production method for the electrochemical measuring sensor 10 willbe explained in greater detail by way of FIG. 2. Parts that areidentical to those in FIG. 1 are provided with identical referencenumerals and will not be explained again. For reasons of clarity, theillustration of the complete measuring sensor 10 was dispensed with.

By way of the perspective plan view, it becomes clear that the solidelectrolyte 12 is essentially plate-shaped. The solid electrolyte 12 iscomprised, for example, of yttrium-stabilized zirconium oxide and isavailable in the form of a film. The electrode 20 is mounted on the side18, here disposed on top, of the solid electrolyte 12. The electrode 20is usually applied by means of known process steps such as, for example,imprinting. Here, the electrode 20 projects beyond the contour of thesolid electrolyte 12. The solid electrolyte 12 as well as the electrode20 and the electrode 16--not visible here--arranged on the opposite sideare still present as so-called green films, that is, these have not beensintered yet.

The electrode 20 has an electrode head 38 which can be connected to acircuit arrangement, not shown, via a conductor track 40. Once theelectrode 20 has been applied to the solid electrolyte 12, embossingtakes place by means of an embossing stamp 42, indicated here, which hasa grid pattern having the later arrangement of the network 28 that isformed by the trench-shaped patterns 26. By applying an embossing forceon the embossing stamp 42, the lattice-shaped pattern of the embossingstamp 42 is imaged in the electrode 20 as well as partially in the solidelectrolyte 12. The embossing force simultaneously presses the electrode20 into the solid electrolyte 12 so that the pattern having theelectrode 20 embedded in the solid electrolyte 12 is formed--as is shownin FIG. 1 in the sectional representation. Once the embossing stamp 42is lifted, the trench-shaped patterns 26 crossing one another are leftin the electrode 20.

Following the embossing process, the entire electrochemical measuringsensor 10 is sintered in a known manner so that the individual layersare tightly bonded to one another. During this process, a stabilizationof the measuring sensor 10 and of the trench-shaped patterns 26 embossedin the measuring sensor 10, particularly in the electrode 20, takesplace at the same time. Here, the trench-shaped patterns 26 are arrangedsuch that, at one face side 44 of the measuring sensor 10, they haveopenings 46, even after they are covered with the cover plate 30illustrated in FIG. 1, so that a reference gas can flow through thechannel network which is formed by the trench-shaped patterns 26.

Overall, the electrochemical measuring sensor 10 has a very compactdesign which is accomplished by means of simple process steps. Duringthis process, the individual patterns of the electrochemical measuringsensor 10 can be accomplished in the so-called panel, that is, aplurality of measuring sensors 10 can be patterned at the same timewhich are separated in an appropriate manner after patterning andsintering. The embossing of the electrode 20 accomplishes that theelectrode 20, in particular, its electrode head 38, is supplied with thereference gas without any problems without necessitating additionalcomplex patterns. Because of the formation of the reference air channelnetwork by the electrode 20 itself, an optional layer 34 with itsheating device 32 can be positioned closer to the sensor section formedby the solid electrolyte 12 with the electrodes 16 and 20, so that thisresults in an improved thermal coupling of the heating device 32. Thispermits a smaller load on the heating device 32 since, for the heatingof the sensor section, intermediate layers no longer have to be heatedas well.

Furthermore, due to the formation of the reference gas channel networkby the electrode 20 itself, the effective electrode surface of theelectrode 20 is enlarged vis-a-vis the reference gas, so that a pumpingoutput of the electrode 20 is improved.

Finally, the contouring of the electrode 20 improves the overallstability of the electrochemical measuring sensor 10. The meander-shapedpatterning of the electrode 20, which results from the embossing of thetrench-shaped patterns 26, simultaneously forms stiffening ribs orstiffening regions which contribute to increasing the strength of theentire electrochemical measuring sensor 10. It is possible, inparticular, to enlarge the electrode 20, in particular the electrodehead 38, in relationship to the solid electrolyte surface, so that themargin regions of the solid electrolyte 12 remaining around theelectrode 20 can be made smaller. Apart from the elimination of theabove-cited intermediate layer for forming the air reference channels, afurther miniaturization of the entire electrochemical measuring sensor10 is hereby possible. Overall, the electrochemical measuring sensor 10may thus be comprised of, for example, merely two films, with a firstfilm being formed by the solid electrolyte 12 with the electrodes 16 and20 and a second film by the cover plate 30 with the layer 34 comprisingthe heating conductors 36.

In the example that is illustrated, the trench-shaped patterns 26 areembossed so as to be essentially square, seen in cross section. Ofcourse, any other cross sectional shape, for example, trapezoid,triangular, semicircular, etc. is suitable.

According to a further embodiment, the electrode 16 can, of course, alsobe embossed in an entirely analogous manner to enlarge the effectiveelectrode surface. This enlarges the surface of the electrode 16 whichis connected to the gas to be measured. Since, during the embossingprocess, the solid electrolyte 12 as well as the electrodes 16 and 20are still present in their pasty form, that is in their green state, acontouring or patterning is possible in any conceivable manner. Thismeans that, for example, by way of a corresponding contouring, theelectrodes 16 or 20 can be "moved" to different horizontal planes of theelectrochemical measuring sensor 10, so that, for the electricalcontacting of the electrodes 16 or 20 line crossings can be implementedin a simple manner. Additionally, through-contacting can be facilitatedbecause, in the embossed regions of the electrodes 16 or 20, thethickness of the solid electrolyte 12 between the correspondingelectrode regions is reduced.

FIG. 3 shows an electrode 48 of an electrochemical measuring sensor inplan view. The electrode 48 shown here is used for electrochemicalmeasuring sensors which are of a different design than the measuringsensor 10 illustrated in FIGS. 1 and 2. The electrodes essentially havethe shape of a circular cylinder and are provided on their surface withcircumferential contouring 24 extending coaxially with respect to acenter point 50, which contours are formed by trench-shaped patterns 26embossed into the electrode 48. If the electrode 48 illustrated in FIG.3 having the surface shown there is exposed to a gas to be measured orto a reference gas, the effective electrode surface, which comes intocontact with the gas to be measured or the reference gas, isconsiderably larger than that of an electrode having an entirely planarsurface. This results in the advantages which were already mentionedabove. Compared to the known electrodes, the electrodes 16, 20 or 48according to the invention have a much higher effective electrodesurface and thus a higher electrode activity while the space requirementremains unchanged and additional material is not used.

We claim:
 1. An electrochemical measuring sensor having a solidelectrolyte, a first electrode exposed to a gas to be measured and asecond electrode exposed to a reference gas, with the electrodes beingarranged on the solid electrolyte, and wherein at least one of theelectrodes has a contouring on its surface that is exposed to the gas,with the contouring forming at least one trench-shaped groove, and withthe trench-shaped groove being embossed into the surface of theelectrode such that the electrode is impressed into the adjacent solidelectrolyte in the region of the trench-shaped groove.
 2. Anelectrochemical measuring sensor according to claim 1, wherein thetrench-shaped groove has openings at least on one end surface of thesolid electrolyte, which openings are connected with the reference gas.3. An electrochemical measuring sensor according to claim 1, wherein aplurality of the trench-shaped grooves are provided and form a networkstructure.
 4. An electrochemical measuring sensor according to claim 1,wherein the electrode provided with the groove is embedded in the solidelectrolyte so that the solid electrolyte and the electrode have aplanar surface.
 5. An electrochemical measuring sensor according toclaim 1, wherein a plurality of the trench-shaped grooves are providedand covered by a cover plate so that a channel network is formed whichhas openings at one end surface of the solid electrolyte.
 6. A methodfor producing an electrochemical measuring sensor having a solidelectrolyte, a first electrode exposed to a gas to be measured and asecond electrode exposed to a reference gas, comprising applying atleast one of the electrodes essentially layer-shaped on the solidelectrolyte in a non-sintered state, contouring the at least one of theelectrodes on its surface that is exposed to a gas by embossing at leastone trench-shaped groove into the electrode surface while the solidelectrolyte and the at least one electrode are in a non-sintered state,and subsequently sintering the solid electrolyte and the at least oneelectrode.
 7. A method according to claim 6, wherein a network ofgrooves that cross one another is embossed into the surface of the atleast one electrode.
 8. A method according to claim 6 wherein theembossing step includes impressing the trench-shaped groove into thesurface of the electrode such that the electrode is impressed into theadjacent solid electrolyte in the region of the trench-shaped groove.